It has been known to plate metal, metal mixtures, and alloys in particulate form on a metal substrate by applying mechanical force sufficient to cause adhesion between the plating metal particles and the surface of the substrate. The mechanical force necessary to cause such adhesion is achieved by placing the plating metal particles, a solid impaction media (e.g. glass or steel beads), materials which promote such plating, and a metal substrate in a rotating ball mill or a tumbling barrel. In this manner, the rotation of the ball mill or the tumbling of the barrel imparts kinetic energy to the impaction media which is transferred to the plating metal particles such that these particles are pounded into the surface of the substrate as a coating.
Compared to other metal plating techniques, mechanical plating has the significant advantage of avoiding hydrogen embrittlement. Mechanical plating forms a porous coating on a substrate which permits hydrogen within the coating to escape, preventing the formation of stress fractures. By contrast, hydrogen is trapped within the non-porous coatings applied by other techniques (e.g. electroplating), making it likely that such stress fractures will form.
The early work in this field of mechanical plating was disclosed in the U.S. Pat. Nos. 2,640,001, 2,640,002, Re. 23,861, 2,689,808, and 2,723,204 all to Clayton et al. Typically, these mechanical plating processes were undertaken in the presence of a plating liquid containing additives to improve the efficiency of plating and/or the quality of the metal deposited. These additives include surfactants, film-forming materials, antifoaming agents, dispersing agents, and corrosion inhibitors. Some of these materials are often added together to the plating liquid as a promoter chemical. For example, U.S. Pat. No. 3,460,977 to Golben discloses promoter chemicals containing specific surfactants and organic acid materials for mechanical plating. U.S. Pat. No. 3,328,197 to Simon teaches utilizing promoter chemicals in the form of a solid cake or bar which contains a combination of mechanical plating promotor chemicals. As the mechanical plating cycle progresses, the bar or cake dissolves at a rate which provides optimal amounts of promoter chemicals to the mechanical plating process. U.S. Pat. No. 3,268,356 to Simon discloses incrementally adding the promoter chemical and/or the plating metal particles to the plating barrel in successive additions to optimize the density and uniformity of the plating metal coating over the entire substrate surface and to provide a substrate surface for subsequently-applied plating particles.
In U.S. Pat. No. 3,400,012 to Golben, the advantages of electroplating were sought to be achieved in a mechanical plating process. Such galvanomechanical plating was effected by adding to the plating liquid a "driving" or plating-inducing metal and an ionizable salt of the metal to be plated. The "driving" metal selected is one which is less noble than the plating metal or the metal of the metallic surfaces to be plated. For example, in mechanically plating tin onto steel washers, the plating metal is in the form of tin chloride, and the driving metal is aluminum powder.
U.S. Pat. No. 3,531,315 to Golben ("'315 patent") discloses performing a mechanical plating process in the presence of a strong acid. Prior to the '315 patent, agitation of the plating metal, the impaction media, and the substrate generally was conducted in the presence of weak organic acids such as citric acid. This required that the contents of the plating barrel be rinsed free of any strong acids used to clean or copper the parts before starting the citric acid-based plating process. With the process of the '315 patent, it is possible to conduct the mechanical plating process without need for intermediate rinsing steps, rendering the process extremely economical.
In German Patent DE 28 54 159 to Tolkmit, intermediate coatings such as the copper flash coating which is normally applied prior to mechanical plating is applied in a single-stage process from a slurry containing an intermediate coating metal and a final metal.
One form of mechanical plating produces a lighter weight, relatively thin coating of 0.1 to 1.0 mils thick. Another form of mechanical plating, often referred to as mechanical galvanizing, results in the application of a thicker (i.e. from about 1.0 to 5.3 mils) and heavier (i.e. from about 0.7 to 2.5 ounces per square foot) mechanically applied metallic coating. During the development of such mechanical galvanizing processes, it was found that enhanced adhesion of mechanical galvanizing coatings could be achieved by building up thin layers of mechanically plated metal.
U.S. Pat. No. 4,389,431 to Erisman ("'431 patent") adapted the process of the '315 patent to the incremental metal powder additions of mechanical galvanizing. This was achieved with two mechanical promoter systems. The first is a flash promoter which coats the substrate with a thin adherent flash coating of a metal more noble than the plating metal prior to adding the plating metal to the system. The second continuing promoter is then incrementally added with some or all of the incremental additions of a finely divided mechanical plating metal, the layers of which are built up to effect mechanical galvanizing.
It is a common practice to lubricate metal parts with lubricating oils or waxes. Lubricants have been applied to threaded and other fasteners to insure that there is adequate clamping force for structural joints. Lubricity can also be improved by codeposition of polytetrafluoroethylene with nickel in an electroless nickel plating process (i.e. chemically plating nickel ions in a bath containing reducing agent by reducing the ions to metallic nickel) as taught by "Electroless Nickel/PTFE Composites --The Niflor Process" by P. R. Ebdon, Int. J. of Vehicle Design, vol. 6, nos. 4-5 (1985), "`Niflor`-- A New Generation Approach to Self Lubricating Surfaces" by P. R. Ebdon, and "Electroless Nickel--PTFE Composite Coatings" by S. S. Tulsi. The resulting coating includes components of the reducing agent (e.g. phosphorus) and is very different than that which can be produced by mechanical plating, because nickel is not soft enough to be mechanically plated and is a barrier material rather than a sacrificial metal like those used in mechanical plating processes such as zinc which corrodes preferentially to the substrate. Alternatively, a lubricating agent has been applied to metals during electroplating by coating a steel substrate with a matrix of zinc or zinc alloys and a fine dispersion of particles consisting of at least one member selected from the group consisting of oxides, carbides, nitrides, borides, phosphides, and sulfides of aluminum, iron, titanium, molybdenum, and copper, as taught by European Pat. No. 174,019, or by codepositing zinc and graphite on metal by electroplating, as taught by "Zinc/Graphite--A Potential Substitute for Antigalling Cadmium" by W. A. Donakowski, Plating and Surface Finishing, vol. 70, p. 48 (1983). Lubricants have also been applied to metal either by anodization and then thermally depositing fluorocarbons or by topcoating a cermet substrate with a fluorocarbon or by creating cracks in a plated coating and then pressing polytetrafluorothylene particles into the cracks, all taught by "Coatings That Are Tough And Slippery", Materials Engineering, Vol. 102 No. 4, pp. 18-20, April 1985. When these methods are used to provide lubrication for fasteners which also require some corrosion resistance, the parts are first plated with a protective, sacrificial coating such as zinc or cadmium prior to application of the lubricant. These last methods require post-treatment steps to apply the lubricant over the protective metal coating.